A technique is disclosed in “Dempa Shinbun Hi-Technology” May 13, 2004 (Non-Patent Document 1) as an example of a technique of integrating a pressure sensor and other dynamic quantity sensors as a module.
FIG. 20 is a diagram illustrating the cross-sectional structure of a sensor die in which a pressure sensor and an acceleration sensor are integrated, as disclosed in Non-Patent Document 1.
The technique disclosed in Non-Patent Document 1 relates to a tire air pressure sensor. In the sensor die illustrated in FIG. 20, a pressure sensor having a function of detecting a tire air pressure and an acceleration sensor having a function of detecting acceleration are integrated into one sensor die. The pressure sensor of FIG. 20 includes a diaphragm Dp that separates a vacuum-sealed space (reference pressure chamber) Kp and air inside a tire, and a piezoelectric resistor element for detecting a deformation of the diaphragm Dp by the tire air pressure is formed on a surface of the diaphragm Dp close to the reference pressure chamber Kp. The acceleration sensor of FIG. 20 is formed in the vacuum-sealed, space Ka different from the reference pressure chamber Kp of the pressure sensor. A force in a radial direction generated in a rotating wheel is detected based on a deformation of a cantilever La, and a wheel module determines whether the wheel rotates or not and the rotation speed thereof.
In the module structure of the pressure sensor and the acceleration sensor illustrated in FIG. 20, since the pressure sensor and the acceleration sensor are formed in the vacuum-sealed spaces Kp and Ka, both sensors can be protected from a lot of chemical substances (residual substances after tire hardening treatment, soap, water, or the like) present inside the tire.
Non-Patent Document 1: “Dempa Shinbun Hi-Technology,” May 13, 2004, Dempa Publications, Inc.
In order to stably control the travelling of a vehicle, high-accuracy dynamic quantity sensors such as a gyro sensor (angular velocity sensor) for detecting an advancing direction of a vehicle as well as a tire air pressure and a rotation speed of a wheel and an acceleration sensor for detecting acceleration in the advancing direction are required. Moreover, since the altitude of the position of a vehicle also changes in accordance with the travelling, a high-sensitivity pressure sensor for detecting a change of atmospheric pressure in accordance with a change of altitude is required. Thus, in recent years, there has been a demand for a compact and inexpensive dynamic quantity sensor device in which a pressure sensor and high-accuracy dynamic quantity sensors such as an acceleration sensor and a gyrosensor are integrated as modules.
In relation to the above demand, the module structure disclosed in Non-Patent Document 1 illustrated in FIG. 20 is configured to detect basically a tire air pressure and a rotation speed of a wheel. The acceleration sensor of the module structure of FIG. 20 is configured to detect a deformation of a cantilever La using a piezoelectric resistor element or the like to thereby detect whether the wheel rotates or not and the rotation speed thereof and is unable to detect the advancing direction of a vehicle or acceleration in the advancing direction with high accuracy.
In the pressure sensor of the module structure of FIG. 20, in order to thin the diaphragm Dp to increase sensitivity, it is generally necessary to form a deep concave portion Hp indicated by a dot line on the rear surface side of a silicon die opposite to the sealed space Kp. The module structure of FIG. 20 is generally manufactured in a wafer state, and the deep concave portion Hp is formed in respective chip formation regions on the rear surface side of the silicon wafer by performing anisotropic etching. However, since the anisotropic etching provides lower processing accuracy in the depth direction than the accuracy in the plane direction, the structure of the pressure sensor of the related art has a problem in that the depth of the concave portion Hp differs in respective chips, and the thickness of the diaphragm Dp becomes uneven.